The 'Battle off Horaniu' was fought between US and Japanese naval forces as a minor naval engagement of the Pacific War and took place near Vella Lavella in the Solomon islands group (17/18 August 1943).

On the night of 17/18 August 1943, a force of four US destroyers intercepted an Imperial Japanese navy convoy carrying troops to Horaniu on the northern coast of Vella Lavella island, where they were to establish a barge base to support the movement of troops through the region. The Japanese convoy was escorted by four destroyers, and both sides exchanged torpedo and gun fire at long range. After two Japanese destroyers had suffered slight damage, the escort withdrew, allowing the US force to sink five of the convoy’s 22 small auxiliary ships and barges. However, the majority of the troop-carrying barges escaped by hiding along the coast of Vella Lavella and subsequently completed their mission on 19 August. In October, the Japanese used the base to support their mass withdrawal of troops from Kolombangara.

In the middle months of 1943, during the aftermath of their successful 'Watchtower' campaign on Guadalcanal island, the Allies launched their next offensive in the Solomon islands group, which was now focused on taking the major Japanese airstrip at Munda on New Georgia island. After their loss of Munda on 4/5 August to 'Toenails' and their naval defeat in the 'Battle of Vella Gulf' on 6/7 August, the Japanese decided to evacuate their garrisons from the central portion of the Solomon islands group, firstly moving to Baanga island while planning to bring forces south from Rabaul on New Britain island for a possible counter-offensive.

On 15 August, the Allies made their 'Dogeared' landing on Vella Lavella island, thereby bypassing the main Japanese troop concentration on Kolombangara island. To maintain contact with these latter troops, and to ensure their later withdrawal, the Japanese planned to establish a staging base at Horaniu, on the northern tip of Vella Lavella island.

Commanded by Rear Admiral Matsuji Ijuin, the destroyers Sazanami, Hamakaze, Isokaze and Shigure departed Rabaul on 17 August to rendezvous with a troop convoy from Buin on Bougainville island. This convoy included 13 barges, four motor torpedo boats, the submarine chasers Ch-12 and Ch-5, and a 'Soukoutei' class armoured boat. The troops embarked on these craft comprised two companies of an Imperial Japanese army unit and one platoon of Imperial Japanese navy troops.

After the Japanese force had been spotted and reported by Allied reconnaissance aircraft, Rear Admiral Theodore S. Wilkinson, commander of the 3^rd Amphibious Force, despatched a division of four destroyers from Purvis Bay on Florida island to effect an interception. This force comprised Nicholas, O’Bannon, Taylor and Chevalier, under the command of Captain Thomas J. Ryan. The US destroyers departed their anchorage at 15.27 on 17 August for a fast run up New Georgia Sound, also known as 'The Slot'. When Ryan’s destroyers were off the northern coast of Kolombangara, look-outs saw a burst of anti-aircraft fire in the distance, giving away the position of the Japanese convoy.

Despite a full moon, the visibility was only 3 miles (4.8 km) as a result of low-lying clouds and intermittent rain showers. At about 23.30, however, the Japanese convoy came under attack by by eight Grumman TBF Avenger single-engined torpedo bombers of Brigadier General Nathan F. Twining’s 'AirSols' (Air Solomons) command, and forced to scatter, although no ships were damaged. Two of the escorts, Isokaze and Shigure, steaming abreast, then began the process of herding the smaller craft back into formation, while Sazanami and Hamakaze continued on a north-westerly course. Ryan was advised by the crew of the Avenger warplanes that the Japanese ships were still heading for Vella Lavella, and his destroyers were detected shortly after this by a Japanese reconnaissance aeroplane, which began to circle in preparation for an attack. The convoy was still re-forming when at 00.29 on 18 August, Ryan’s radar detected the Japanese destroyers to the north-west at a range of 23,000 yards (21030 m), then detected the Japanese barges. At 00.32 Japanese look-outs spotted the US destroyers, which had closed to a range of 16,400 yards (14995 m).

The Japanese convoy was still 16 miles (26 km) short of its destination, and Ijuin was under orders to avoid any potentially decisive engagement. He therefore ordered two of his destroyers to turn to the north in an effort to lure the US force away from the troop-carrying vessels. To conceal his position, Ryan decided not to use his deck guns, and began manoeuvring his destroyers for a torpedo attack on the Japanese destroyers, rather than attacking the barges. This surprised Ijuin, who had expected exactly the opposite. At about 00.40, the circling Japanese aeroplane dropped several flares over the US ships, revealing their location, and Ryan turned back to engage the convoy to his east. Six minutes later, Ijuin ordered his ships to begin firing torpedoes, of which 31 were launched between 00.50 and 00.54 at a range of 12,500 yards (11430 m). By making a series of turns to avoid the Japanese barges, the US destroyers inadvertently avoided all 31 torpedoes, and at about 00.56 the Japanese gunners opened fire. To avoid revealing their position, however, the Japanese destroyers' searchlights were not illuminated, and no damage was inflicted on Ryan’s destroyers.

After a series of turns, the US destroyers opened fire at 00.58 with their radar-controlled 5-in (127-mm) main guns. These damaged Hamakaze, while Chevalier loosed a salvo of four torpedoes towards Shigure at a range of 9,000 yards (8230 m). This attack was unsuccessful. The Japanese ships laid a smokescreen and began to zigzag in an attempt to throw off the US gunners' aim. Torpedo attacks by both Shigure and Isokaze were also unsuccessful, and was followed by an inaccurate radar report that another US naval force was approaching from the south. Ijuin could not afford to lose any of his destroyers, and after an ineffective long-range exchange of torpedoes and gunfire, ordered his ships to withdraw at about 01.00. Ryan’s ships kept firing until the Japanese were out of range, then pursued them, moving onto a parallel course, while undertaking evasive manoeuvres until 01.11 to avoid any torpedoes that might have been fired. At this time, Isokaze fired from a range of 14,215 yards (13000 m) a torpedo salvo which failed to score a hit, but in return was damaged by a 5-in (127-mm) shell, whose detonation wounded several sailors.

As a result of a mechanical fault on Chevalier, the US force was unable to catch the fast-moving Japanese destroyers, which were withdrawing at 35 kt. Ryan turned back to locate the troop convoy, but the small craft had taken advantage of the destroyer contest to disperse. Although the Americans managed to sink five of the small craft, the majority escaped. Harassed by the Japanese reconnaissance aeroplane, Ryan’s destroyers then returned to Tulagi island.

The five Japanese auxiliary vessels sunk included the two submarine chasers, two motor torpedo boats and one powered barge with an unknown number of personnel killed or injured. In addition, two destroyers, Hamakaze and Isokaze, had suffered slight damage, although this was not significant and both took part in further actions three days later. Despite these losses, the Japanese saved the majority of their barges, which spent 18 August lying quietly in hiding along the northern coast of Vella Lavella island, and were subsequently able to land 390 troops on 19 August. These personnel established a barge base at Horaniu, while Ijuin’s force returned to Rabaul believing that it had sunk one US destroyer.

The Japanese withdrawal in the central part of the Solomon islands group continued throughout August and September. After Baanga island had been evacuated on 22 August, the Japanese garrison withdrew to Arundel island, and over the course of a month fought a series of delaying actions there until a time late in September. Meanwhile, on Vella Lavella island, Horaniu fell to Allied forces on 14 September, and the base’s 600 personnel were pushed into a small perimeter on the island’s north-western coast. From this they were withdrawn on the night of 6/7 October, together with almost 10,000 men who were evacuated by sea from Kolombangara island to Choisel and Bougainville islands.